The invention relates to an optical multiplexer/demultiplexer, particularly to an optical multiplexer/demultiplexer which can significantly reduce the wavelength dispersion, and further to an optical multiplexer/demultiplexer which can simultaneously perform multiplexing processing and demultiplexing processing.
An interleave system, which is one form of advanced wavelength multiplexing communications, requires an optical multiplexer/demultiplexer having a function such that a signal with certain channel wavelength spacings is demultiplexed to two signals with doubled channel wavelength spacings, or conversely, two signals are multiplexed to one signal.
FIG. 12 is an explanatory view showing one example of a prior art technique for coping with this demand. Since a broad and flat wavelength passband is required of the optical multiplexer/demultiplexer used in the interleave, as shown in FIG. 12, the prior art technique has adopted a multistage-connected construction of Mach-Zehnder interference circuits which each comprise four optical couplers (directional couplers) 24, 25, 26, 27 and waveguide pairs each comprising two waveguides with different lengths (28, 29), (30, 31), (32, 33), for connecting the optical couplers to each other (see, for example, U.S. Pat. No. 5,852,505).
FIG. 13 shows a spectral response for an input/output port 22 in the case where a white light source is input through an input/output port 21, and FIG. 14 a spectral response for an input/output port 23. When signals 1, 2, 3, 4 with channel spacings of about 0.4 nm (frequency 50 GHz) are input through the port 21, 1 and 3 are output through the port 22 while 2 and 4 are output through the port 23. In this case, the output signals have channel wavelength spacings of about 0.8 nm (frequency 100 GHz). The multistage construction of Mach-Zehnder interference circuits shown in FIG. 12 has an advantage that, as shown in wavelength loss characteristics in FIGS. 13 and 14, a broad and flat passband can be provided.
The optical multiplexer/demultiplexer shown in FIG. 12, however, suffers from a problem that the realization of good loss wavelength flatness disadvantageously leads to wavelength dispersion.
FIG. 15 shows wavelength dispersion characteristics for a path wherein optical signals are input through the port 21 and output through the port 22, and FIG. 16 wavelength dispersion characteristics for a path wherein optical signals are input through the port 21 and output through the port 23. For both the drawings, in the abscissa, frequency is used instead of the wavelength, while the ordinate represents only dispersion around passbands (around 1 passband and around 2 passband). As is apparent from the drawings, the dispersion around the passbands is about 30 ps/nm. This value, of course, varies depending upon parameters. In theory, however, it is unavoidable that improving the loss wavelength flatness leads to dispersion. This wavelength dispersion is significantly disadvantageous in terms of transmission speed of the system and relay distance.
Further, the conventional optical multiplexer/demultiplexer can have both optical multiplexing and optical demultiplexing functions. However, multiplexing processing and demultiplexing processing of a plurality of optical signals derived from different signal sources could not have been simultaneously performed in a single optical multiplexer/demultiplexer. This has necessitated the provision of at least one optical multiplexer/demultiplexer for multiplexing and at least one optical multiplexer/demultiplexer for demultiplexing. Therefore, a space for the necessary number of optical multiplexer/demultiplexers should be provided.
Accordingly, it is an object of the invention to solve the above problem of the prior art and to provide an optical multiplexer/demultiplexer which causes no significant wavelength dispersion (theoretically has no wavelength dispersion) and, at the same time, has excellent wavelength flatness characteristics in passband.
It is another object of the invention to realize an optical multiplexer/demultiplexer which can reduce the space necessary for the optical multiplexer/demultiplexer to perform the multiplexing processing and demultiplexing processing of a plurality of optical signals derived from different signal sources.
According to the first feature of the invention, an optical multiplexer/demultiplexer comprises optical multiplexer/demultiplexer circuits connected in multistage, wherein:
the optical multiplexer/demultiplexer circuits each have two input ports and two output ports;
an optical path, through which input optical signals with predetermined wavelengths are output after multiplexing and demultiplexing, varies depending upon the input port;
the optical paths in each of the optical multiplexer/demultiplexer circuits have mutually opposite wavelength dispersion characteristics; and
one of the optical paths in a first optical multiplexer/demultiplexer circuit is connected to one of the optical paths, in a second optical multiplexer/demultiplexer circuit, having wavelength dispersion characteristics opposite to the optical path in the first optical multiplexer/demultiplexer circuit.
This optical multiplexer/demultiplexer may comprise two optical multiplexer/demultiplexer circuits connected to each other, wherein:
said optical multiplexer/demultiplexer circuits each have a waveguide with first and third ports as terminals and a waveguide with second and fourth ports as terminals, and have optical multiplexing/demultiplexing characteristics such that, for optical signals with predetermined wavelengths, the optical signals input through the first port are output through the third port while optical signals input through the second port are output through the fourth port;
the wavelength dispersion characteristics, in the case where the optical signals are input through the first port and output through the third port, are opposite to the wavelength dispersion characteristics in the case where the optical signals are input through the second port and output through the fourth port; and
the third port in the first optical multiplexer/demultiplexer circuit, when the first port in the first optical multiplexer/demultiplexer circuit is used as an input port, is connected to the second port in the second optical multiplexer/demultiplexer circuit while the fourth port in the second optical multiplexer/demultiplexer circuit is used as an output port for optical signals with predetermined wavelengths.
Further, the optical multiplexer/demultiplexer may comprise two optical multiplexer/demultiplexer circuits connected to each other, wherein:
said optical multiplexer/demultiplexer circuits each have a waveguide with first and third ports as terminals and a waveguide with second and fourth ports as terminals, and have optical multiplexing/demultiplexing characteristics such that, for optical signals with predetermined wavelengths, the optical signals input through the first port are output through the fourth port while optical signals input through the second port are output through the third port;
the wavelength dispersion characteristics, in the case where the optical signals are input through the first port and output through the fourth port, are opposite to the wavelength dispersion characteristics in the case where the optical signals are input through the second port and output through the third port; and
the fourth port in the first optical multiplexer/demultiplexer circuit, when the first port in the first optical multiplexer/demultiplexer circuit is used as an input port, is connected to the second port in the second optical multiplexer/demultiplexer circuit while the third port in the second optical multiplexer/demultiplexer circuit is used as an output port for optical signals with predetermined wavelengths.
Furthermore, the optical multiplexer/demultiplexer may comprise three optical multiplexer/demultiplexer circuits integrated with each other, wherein:
said optical multiplexer/demultiplexer circuits each have a waveguide with first and third ports as terminals and a waveguide with second and fourth ports as terminals;
said optical multiplexer/demultiplexer circuits have optical multiplexing/demultiplexing characteristics such that, for optical signals with predetermined wavelengths, the optical signals input through the first port are output through the third port while optical signals input through the second port are output through the fourth port, and the wavelength dispersion characteristics, in the case where the optical signals are input through the first port and output through the third port, are opposite to the wavelength dispersion characteristics in the case where the optical signals are input through the second port and output through the fourth port;
for other optical signals with predetermined wavelengths, the optical multiplexing/demultiplexing characteristics are such that the optical signals input through the first port are output through the fourth port while optical signals input through the second port are output through the third port, and the wavelength dispersion characteristics, in the case where the optical signals are input through the first port and output through the fourth port, are opposite to the wavelength dispersion characteristics in the case where the optical signals are input through the second port and output through the third port;
the third port in the first optical multiplexer/demultiplexer circuit, when the first port in the first optical multiplexer/demultiplexer circuit is used as an input port, is connected to the second port in the second optical multiplexer/demultiplexer circuit, and the fourth port in the first optical multiplexer/demultiplexer circuit is connected to the second port in the third optical multiplexer/demultiplexer circuit; and
the fourth port in the second optical multiplexer/demultiplexer circuit is used as an output port for optical signals with predetermined wavelengths while the third port in the third optical multiplexer/demultiplexer circuit is used as an output port for other optical signals with predetermined wavelengths.
Furthermore, the optical multiplexer/demultiplexer may comprise two optical multiplexer/demultiplexer circuits connected to each other, wherein:
said optical multiplexer/demultiplexer circuits each have a waveguide with first and third ports as terminals and a waveguide with second and fourth ports as terminals, and have optical multiplexing/demultiplexing characteristics such that, for optical signals with predetermined wavelengths, the optical signals input through the first port are output through the third port while optical signals input through the second port are output through the fourth port;
the wavelength dispersion characteristics, in the case where the optical signals are input through the first port and output through the third port, are opposite to the wavelength dispersion characteristics in the case where the optical signals are input through the second port and output through the fourth port; and
the third port in the first optical multiplexer/demultiplexer circuit, when the first port in the first optical multiplexer/demultiplexer circuit is used as an input port, is connected to the fourth port in the second optical multiplexer/demultiplexer circuit while the second port in the second optical multiplexer/demultiplexer circuit is used as an output port for optical signals with predetermined wavelengths.
Furthermore, the optical multiplexer/demultiplexer may comprise two optical multiplexer/demultiplexer circuits connected to each other, wherein:
said optical multiplexer/demultiplexer circuits each have a waveguide with first and third ports as terminals and a waveguide with second and fourth ports as terminals, and have optical multiplexing/demultiplexing characteristics such that, for optical signals with predetermined wavelengths, the optical signals input through the first port are output through the fourth port while optical signals input through the second port are output through the third port;
the wavelength dispersion characteristics, in the case where the optical signals are input through the first port and output through the fourth port, are opposite to the wavelength dispersion characteristics in the case where the optical signals are input through the second port and output through the third port; and
the fourth port in the first optical multiplexer/demultiplexer circuit, when the first port in the first optical multiplexer/demultiplexer circuit is used as an input port, is connected to the third port in the second optical multiplexer/demultiplexer circuit while the second port in the second optical multiplexer/demultiplexer circuit is used as an output port for optical signals with predetermined wavelengths.
Furthermore, the optical multiplexer/demultiplexer may comprise three optical multiplexer/demultiplexer circuits integrated with each other, wherein:
said optical multiplexer/demultiplexer circuits each comprise a waveguide with first and third ports as terminals and a waveguide with second and fourth ports as terminals;
said optical multiplexer/demultiplexer circuits have optical multiplexing/demultiplexing characteristics such that, for optical signals with predetermined wavelengths, the optical signals input through the first port are output through the third port while optical signals input through the second port are output through the fourth port, and the wavelength dispersion characteristics, in the case where the optical signals are input through the first port and output through the third port, are opposite to the wavelength dispersion characteristics in the case where the optical signals are input through the second port and output through the fourth port;
for other optical signals with predetermined wavelengths, the optical multiplexing/demultiplexing characteristics are such that the optical signals input through the first port are output through the fourth port while optical signals input through the second port are output through the third port, and the wavelength dispersion characteristics, in the case where the optical signals are input through the first port and output through the fourth port, are opposite to the wavelength dispersion characteristics in the case where the optical signals are input through the second port and output through the third port;
the third port in the first optical multiplexer/demultiplexer circuit, when the first port in the first optical multiplexer/demultiplexer circuit is used as an input port, is connected to the fourth port in the second optical multiplexer/demultiplexer circuit, and the fourth port in the first optical multiplexer/demultiplexer circuit is connected to the third port in the third optical multiplexer/demultiplexer circuit; and
the second port in the second optical multiplexer/demultiplexer circuit is used as an output port for optical signals with predetermined wavelengths while the second port in the third optical multiplexer/demultiplexer circuit is used as an output port for other optical signals with predetermined wavelengths.
The optical multiplexer/demultiplexer circuits are preferably constructed so that Mach-Zehnder interference circuits each comprising a plurality of optical couplers, including directional couplers, and two waveguides, with different lengths, connecting the optical couplers to each other are connected in multistage.
Further, the Mach-Zehnder interference circuits each preferably comprise a quartz-based plane optical waveguide substrate. In this case, since three Mach-Zehnder interference circuits can be integrated on a single substrate, the size of the optical multiplexer/demultiplexer can be reduced as compared with the case where three Mach-Zehnder interference circuits are connected through an optical fiber.
Still further, according to another feature of the invention, an optical multiplexer/demultiplexer comprises: first, second, and third optical multiplexer/demultiplexer circuits each having a plurality of optical paths including waveguides; and first, second, and third optical isolators, wherein
said optical multiplexer/demultiplexer circuits each have first, second, third, and fourth ports respectively at the terminals of the optical paths, and the first and second ports are connected to the third and forth ports through the optical paths including waveguides,
an optical signal with a first predetermined wavelength, when input into the first port, is passed through the first optical path and is output through the third port; and the optical signal with the first wavelength, when input into the fourth port, is passed through the second optical path and is output through the second port, the wavelength dispersion characteristics of the first optical path being opposite to the wavelength dispersion characteristics of the second optical path with respect to the optical signal with a first wavelength,
an optical signal with a second predetermined wavelength, when input into the first port, is passed through the third optical path and is output through the fourth port; the optical signal with the second wavelength, when input into the fourth port, is passed through the third optical path and is output through the first port; and the optical signal with the second wavelength, when input into the third port, is passed through the fourth optical path and is output through the second port, the wavelength dispersion characteristics of the third optical path being opposite to the wavelength dispersion characteristics of the fourth optical path with respect to the optical signal with a second wavelength,
said first optical isolator is connected to the first port, on its input side, of the first optical multiplexer/demultiplexer circuit so that the optical signal with the first or second wavelength is input into the first optical multiplexer/demultiplexer circuit through the first optical isolator,
said second optical isolator is connected to the first port, on its external side, of the second optical multiplexer/demultiplexer circuit, and the third optical isolator is connected to the first port, on its external side, of the third optical multiplexer/demultiplexer circuit,
the third port of the first optical multiplexer/demultiplexer circuit is connected to the fourth port of the second optical multiplexer/demultiplexer circuit, while the fourth port of the first optical multiplexer/demultiplexer circuit is connected to the third port of the third optical multiplexer/demultiplexer circuit,
said optical signal with the first wavelength, when input into the first port of the first optical multiplexer/demultiplexer circuit through the first optical isolator, is passed through the first optical path of the first optical multiplexer/demultiplexer circuit and the second optical path of the second optical multiplexer/demultiplexer circuit to perform optical multiplexing/demultiplexing, followed by the output of the multiplexed/demultiplexed signal through the second port of the second optical multiplexer/demultiplexer circuit,
said optical signal with the first wavelength, when input into the first port of the third optical multiplexer/demultiplexer circuit through the third optical isolator, is passed through the first optical path of the third optical multiplexer/demultiplexer circuit and the second optical path of the first optical multiplexer/demultiplexer circuit to perform optical multiplexing/demultiplexing, followed by the output of the multiplexed/demultiplexed signal through the second port of the first optical multiplexer/demultiplexer circuit,
said optical signal with the second wavelength, when input into the first port of the first optical multiplexer/demultiplexer circuit through the first optical isolator, is passed through the third optical path of the first optical multiplexer/demultiplexer circuit and the fourth optical path of the third optical multiplexer/demultiplexer circuit to perform optical multiplexing/demultiplexing, followed by the output of the multiplexed/demultiplexed signal through the second port of the third optical multiplexer/demultiplexer circuit, and
said optical signal with the second wavelength, when input into the first port of the second optical multiplexer/demultiplexer circuit through the second optical isolator, is passed through the third optical path of the second optical multiplexer/demultiplexer circuit and the fourth optical path of the first optical multiplexer/demultiplexer circuit to perform optical multiplexing/demultiplexing, followed by the output of the multiplexed/demultiplexed signal through the second port of the first optical multiplexer/demultiplexer circuit.